1. Field of the Invention
The invention relates to a method of projecting a mask pattern on a plurality of fields of a substrate, having a radiation-sensitive layer, by means of a projection beam and a projection system, and wherein prior to introducing a substrate holder with the substrate into the projection beam and underneath the projection system, the surface profile of the substrate is determined, for each substrate field, by:
measuring the height in a direction parallel to the axis of the projection beam; PA1 measuring the height of a reference plane of the substrate holder; PA1 establishing a relation between the height of the substrate field and the height of the reference plane of the substrate holder, and PA1 storing this relation in a memory, and, after introducing the substrate holder with a substrate into the projection beam for illuminating each substrate field, the height of this field is adjusted by checking the height of the reference plane of the substrate holder.
The invention also relates to a product manufactured by means of the method and to a lithographic projection apparatus which is suitable for performing the method. This apparatus may be a stepper or a step-and-scanner.
2. Description of the Related Art
A method and apparatus of this type for manufacturing ICs is described in published Japanese patent application JP-A 61-196532. This patent application mentions the problem that, during the IC manufacturing process, the surface of the substrates may be deformed because these substrates undergo a thermal treatment between consecutive illuminations, each time with a different mask pattern. Due to such a deformation of the substrate surface, IC areas or fields of the substrate may get beyond the depth of focus of the projection lens system with which the mask pattern is imaged on the different substrate fields, so that no good images can be realized any longer. It is therefore necessary to measure the height of each substrate field, i.e. the position along an axis parallel to the optical axis of the projection lens system. If a height measurement is performed at a point for each substrate field, a possible unevenness of the relevant substrate field cannot be measured and it cannot be ensured that the entire substrate field is within the depth of focus of the projection lens system.
The object of JP-A 61-196532 is to provide a method with which it can be ensured that the entire substrate field surface is within the depth of focus and with which it can be ascertained whether this surface has a quality which is too bad to be illuminated or should not be used at all. To realize this object, JP-A 61-196532 proposes to measure the height and possible tilt of the fields of the substrates before they are introduced into the projection beam and underneath the projection lens system, hence in a projection station. This measurement is performed in a separate measuring station.
An important parameter of the current lithographic apparatuses is the throughput, i.e. the number of substrates which can be illuminated per unit of time by the apparatus, hence provided with images of the mask pattern. As is known, there is a very rapid development in the field of lithographic apparatuses. After the publication of JP-A 61-196532, more generations of these apparatuses have meanwhile succeeded each other. With the successive generations, ICs having an increasing number of electronic components could be manufactured. However, this meant that the procedures for aligning (in an X and Y direction perpendicular to the axis of the projection system) of the substrate fields with respect to the mask pattern and for keeping these fields in focus became more difficult and more time-consuming. During this alignment, not only an alignment system in which alignment marks of the substrate are imaged on the marks of the mask and which is described in, for example U.S. Pat. No. 4,778,275, but also an interferometer system is used with which the X and Y movements of the substrate and the positions of the substrate fields can be fixed in a system of co-ordinates.
An important breakthrough was the use of an interferometer system with at least five instead of three measuring axes. A lithographic apparatus provided with such an interferometer system is described in EP-A 0 498 499. With this interferometer system, not only the displacements of the substrate along the X axis and the Y axis and the rotation about the Z axis can be measured, but also the tilt about the X axis and the tilt about the Y axis can be measured very accurately. Consequently, each substrate field can be positioned with sufficient accuracy with respect to the mask pattern without separate alignments having to be performed per substrate field. The time required for illuminating the substrate can thus be reduced considerably.
In JP-A 61-196532, a separate measuring station is used for measuring the height and the tilt of the substrate fields. It is further remarked that, due to the use of a plurality of substrate holders, the illumination of a first substrate and the measurement on a second substrate can be performed simultaneously so that the same throughput as in known apparatuses can be achieved. The reason behind this is that, due to the various and extra measuring steps to be performed per substrate field, the time required for illuminating the entire substrate would become too long if the apparatus treated substrate by substrate, i.e. first consecutively measuring and illuminating a first substrate, subsequently consecutively measuring and illuminating a second substrate, and so forth. However, due to the facilities provided by the interferometer system with at least five measuring axes, the problem and solution mentioned in JP-A 61-196532 have been superseded, and the accuracy aimed at can also be achieved without a separate measuring station and without a parallel-time measurement and illumination.
For the novel lithographic apparatuses and the lithographic apparatuses which are currently being developed, with which even smaller details must be imaged and an even greater positioning accuracy of the substrate fields is desirable, it is necessary to align per substrate field and to perform a focus and tilt correction despite the use of an interferometer system with five or more measuring axes.
It has already been proposed, for example in EP-A 0 687 957 and the English-language abstract of JP-A 57-183031 to provide a lithographic projection apparatus with at least two substrate holders and a separate alignment station. In this station, the substrates are aligned with respect to the substrate holder on which they are present, prior to their introduction into the projection station. In the projection station, only the substrate holder is then to be aligned with respect to the mask pattern, which is a relatively simple process which can be performed rapidly. Since two substrate holders are used, which can be moved between the alignment station and the projection station, a second substrate can be aligned with respect to the substrate support in the alignment station while a first substrate is being illuminated in the projection station, and the time required for alignment in the projection station can be minimized.
In the height-measuring station described in JP-A 61-196532, the same multiple height sensor consisting of, for example three air sensors is used for measuring the height of both a substrate field and a reference plane of the substrate holder. Furthermore, a shearing interferometer system is provided for measuring the shape of the substrate field surface. The result of this choice of measuring devices is that the measuring procedure comprises a relatively large number of steps.
First, the height of the substrate field is measured at three different positions in this field by means of the three air sensors, so that the slope of this field can be computed. This slope is referred to as "temporary base plane". Subsequently, it is ensured by means of a vertical actuator which is present in the height-measuring station that the temporary base plane is parallel to a reference plane of the interferometer. Then, the air sensors of the substrate surface are moved to the reference plane of the substrate holder. Subsequently, the substrate field is measured with the interferometer, while the shape of the substrate field surface is computed from the interference pattern formed in the interferometer system. During this step the substrate must be moved vertically over small distances by the vertical actuator. Subsequently, the height of the substrate support reference plane is measured with the three air sensors. Finally, the correlation between this height and the temporary base plane is determined. The information thus obtained is sent to the projection station where it is used, after arrival of the relevant substrate, for adjusting the height of the substrate fields, at which only the height of the substrate reference plane is measured with three air sensors which are present in the projection station.